Sensors for determining the degree of roughness by optical means do not provide generally the amount of information available from the device of the present invention. Also discrimination between a dirt particle and a topographical feature is sometimes difficult.
Devices have been described in the patent literature, for example, Canadian Pat. No. 617,188 issued March 28, 1967 discloses a device for detecting surface conditions on a travelling web. This device is sensitive to longitudinal striations on the surface of the web and operates by directing a light beam obliquely onto the surface in a direction generally perpendicular to the length of the striations on the web and sensing the amount of light reflected.
Canadian Pat. No. 836,242 issued Mar. 10, 1970 to Joyce discloses a device for determining surface characteristics of a paper sheet in particular for determining a wire mark on a paper sheet. One of the concepts of this invention is to project a light through a web and to use a mask, having a plurality of apertures corresponding at least partially to the pattern of the wire mark, in combination with detecting means which generates signals and to process the signals to obtain an indication, for example, of web speed. The device does not convey general information of the topographical configuration of the web.
U.S. Pat. No. 3,591,291 discloses a device for detecting the roughness of a relatively smooth surface by directing a collimated beam of light against the surface and mounting one light sensitive element at the angle of reflection of said beam from the surface, and further light sensitive elements in the same plane as the illuminating and reflected beams but at different angles to the surface. The signal generated by the light sensitive element located at the angle of reflection is compared with signal developed by one of the other light sensitive elements to give an indication of surface roughness. This device is based on the assumption that the amount of light diffused and diffracted from the peaks on the smooth surface is a function of the inclination of the slopes of those peaks and that those inclinations are functions of the height of the peaks and therefore that the amount of diffusion and diffraction of the light by the peaks on a relatively smooth surface is a measure of the height of the peaks. The device is operated by positioning it on the surface and comparing the light signals with no relative movement between the surface being measured and the sensor.
In an earlier device invented by the inventors of the instant application, the roughness of a moving surface was sensed by projecting a beam of light at a low angle of an inclination to sequentially illuminate small areas on the surface, collecting scattered light reflected from the small areas, generating electrical signals in accordance with the amount of light collected and analyzing these signals to provide, for example an index of roughness. This device is not in general suited for discriminating topography from dirt particles.